1. Field of the Invention
The present invention relates to a pneumatic tire having at the tread thereof blocks in which sipes are formed, and in particular, to a pneumatic tire in which irregular wear is suppressed and noise at the time of wear is decreased.
2. Description of the Related Art
Generally, in pneumatic tires provided with block patterns, irregular wear known as heel-and-toe wear, results in steps due to wear at the leading (step-in) edges and the trailing (kick-out) edges of blocks, and tends to occur. Such wear is accompanied by a deterioration in external appearance and noise, and there is a definite need to solve this drawback.
With heel-and-toe wear, a step is formed due to earlier wear at the toe side which is the trailing edge, than at the heel side which is the leading edge.
An example of a method for improving heel-and-toe wear is a method in which the angle of the groove wall surfaces with respect to a normal line which is orthogonal to the direction in which the groove extends, is changed at the leading edge and the trailing edge.
However, it is difficult to greatly change the angle of the groove wall surfaces due to problems such as removing the tire from a mold at the time of vulcanization or the like. Further, as wear progresses, the width of the groove has a distribution which differs greatly than that at the time the tire was new, which is not preferable from the standpoint of performance nor from the standpoint of external appearance.
Further, as can be seen from Japanese Patent Application Laid-Open (JP-A) No. 2-24204, an idea has been proposed in which sipes are disposed parallel to the contour line at the leading edge of the ground-contact configuration. However, this structure can only be applied to patterns having a specific orientation, and is not practical.
In view of the aforementioned, an object of the present invention is to provide a pneumatic tire in which heel-and-toe wear is suppressed and the noise due to the heel-and-toe wear which is generated is reduced.
Generally, blocks at the shoulder sides of a pneumatic tire are displaced toward the center at the time of step-in, and thereafter, contact the road surface, and at the time of kick-out, return while sliding toward the outer sides, such that wear is generated at these times.
The magnitude of the irregular wear is affected by the magnitude of the shearing strain between the region which first returns (the kicked-out region) and the region which is still contacting the road surface.
In a block pattern, due to the shearing strain between a leading edge (heel side) which is a preceding trailing edge region and a trailing edge (toe side) which is a following trailing edge region, the amount of wear of the trailing edge (the following trailing edge region) is greater than the amount of wear of the leading edge (the preceding trailing edge region), and thus heel-and-toe wear is generated.
As the result of various intensive experimental studies, the present inventors found that in order to mitigate the effect of the shearing strain from the preceding trailing edge region, it is effective to utilize sipes matching the ground-contact configuration at the time of kick-out, i.e., in order to mitigate the affect of the shearing strain from the leading edge side, it is effective to use a sipe at the trailing edge side.
Further, it is known that when heel-and-toe wear is generated at shoulder side blocks, the noise generated at the time the blocks contact the ground becomes large.
In order to determine the cause behind this phenomenon, the force which the ground-contact surface receives from the road surface and the ground-contact configuration were observed in detail, and the following was learned.
As heel-and-toe wear is generated, the force received from the road surface increases at the leading edges of the blocks at the shoulder portions. This is because, due to the heel-and-toe wear generated at the shoulder portions, at the time of step-in of the next block, the trailing edge side of the block of the previous pitch (the block which has already been stepped into) becomes a stepped concave portion. Therefore, the load of the block of the previous pitch is reduced, or in some cases, a region which does not contact the ground arises. This effect increases the ground-contact pressure of the leading edge of the block of the next pitch, and the striking noise at the time of stepping into the block increases.
Accordingly, in order to suppress the increase in ground-contact pressure at the leading edge, it is effective to reduce the stepped concave portion itself and simultaneously to divide the stepped concave portion to diminish the effect on the next pitch.
As a result of various experimental studies, the present inventors learned that the amount of the step due to heel-and-toe wear of an entire block could be effectively suppressed by forming, at the trailing edge side, a sipe conforming to the ground-contact configuration at the time of kick-out.
A first aspect of the present invention is a pneumatic tire having a tread pattern including a plurality of blocks defined by a plurality of circumferential direction grooves extending substantially along a tire circumferential direction and a plurality of transverse direction grooves extending substantially along a tire transverse direction, wherein a sipe, which is substantially parallel to a contour line at a trailing edge of a ground-contact configuration, is formed in a region at a trailing edge of each of blocks at shoulder sides of said pneumatic tire among the plurality of blocks.
The term xe2x80x9cground-contact configurationxe2x80x9d as used herein is defined as follows. In accordance with the 1996 JATMA Year Book (xe2x80x9cJATMAxe2x80x9d stands for the xe2x80x9cJapan Automobile Tire Manufacturers"" Association, Inc.), a tire is rim-assembled to a standard rim at an applied size, and the tire is filled with air pressure corresponding to the maximum load capacity at the applied size/ply rating in accordance with the 1996 JATMA Year Book. The ground-contact configuration is measured when there is a load of 80% of the maximum load capacity at the applied size/ply rating in accordance with the 1996 JATMA Year Book (where there is both description of a single tire and plural tires, the description of the single tire is applicable).
In the pneumatic tire of the first aspect of the present invention, a sipe which is substantially parallel to the contour line at the trailing edge of he ground-contact configuration is provided in a region at the trailing edge of each block at the shoulder side. Therefore, from the time of step-in to the time of kick-out of a block, the effect of the shearing strain transmitted to the trailing edge from the preceding trailing edge region is mitigated, the movement of the trailing edge with respect to the road surface is suppressed, and wear at the trailing edge (the toe side), i.e., heel-and-toe wear, is suppressed.
Instead of heel-and-toe wear (i.e., instead of a large concave portion being formed), minute concave portions are formed in vicinities of the front and rear of the sipe and at the trailing edge of the block. The effect of the stepped concave portion on the surroundings is proportional to the surface area of the region, and is inversely proportional to the distance from the concave portion.
Accordingly, by providing a sipe as described above and by reducing and dividing the stepped concave portion, the degree of increase in the ground-contact pressure of the leading edge of the block of the next pitch is decreased, and the striking noise at the time of step-in of the block can be decreased.
By setting the sipe disposed at the trailing edge region substantially parallel to the contour line at the trailing edge, the effect of the shearing strain from the preceding trailing edge region can be effectively mitigated.
In accordance with a second aspect of the present invention, there is provided a pneumatic tire in which a sipe, which is substantially parallel to the contour line of the leading edge of the ground-contact configuration, is provided at a leading edge region of each block at the shoulder side.
The smaller the compressive rigidity of the leading edge, the more the rate of increase in the ground-contact pressure can be decreased. Accordingly, by providing a sipe, which is substantially parallel to the contour line at the leading edge of the ground-contact configuration, at the leading edge region, the tread rubber deforms easily and the compressive rigidity can be decreased at the leading edge side. Therefore, the striking noise at the time of step-in of the block can be decreased.